1. Field of the Invention
This invention relates to a power processor circuit and method for supplying power to generate corona discharge in a corona discharge pollutant destruction corona discharge reactor.
2. Description of the Related Art
Passing a pollutant bearing gas through a corona discharge site is a known method of removing the pollutants from the gas. A general review of this technique is provided in Puchkarev et al., "Toxic Gas Decomposition by Surface Discharge," Proceedings of the 1994 International Conf. on Plasma Science, Jun. 6-8, 1994, Santa Fe, N.M., paper No. 1E6, page 88. Corona pollutant destruction has also been proposed for liquids, as disclosed in application Ser. No. 08/295,959, filed Aug. 25, 1994, now U.S. Pat. No. 5,549,795, "Corona Source for Producing Corona Discharge and Fluid Waste Treatment with Corona Discharge," and assigned to Hughes Aircraft Company, now doing business as Hughes Electronics.
In one system, described in Yamamoto et al., "Decomposition of Volatile Organic Compounds by a Packed Bed Reactor and a Pulsed-Corona Plasma Reactor," Non-Thermal Plasma Techniques for Pollution Control, NATO ASI Series Vol. G34 Part B, Ed. by B. M. Penetrante and S. E. Schultheis, Springer-Verlag Berlin Heidelberg, 1993, pages 87-89, brief high voltage pulses of about 120-130 nanoseconds duration are applied to the center conductor of a coaxial corona reactor through which gas is flowing. Each pulse produces a corona discharge that emanates from the center wire and floods the inside volume of the reactor with energetic electrons at about 5-10 keV. A similar system is described in U.S. Pat. No. 4,695,358, in which pulses of positive DC voltage are superimposed upon a DC bias voltage to generate a streamer corona for removing SO.sub.x and NO.sub.x from a gas stream. These processes have relatively poor energy efficiencies. With the reactor geometries that have been selected, it is necessary to deliver very short pulses to avoid arc breakdown between the electrodes, and consequent damage. The pulse-forming circuit loses approximately half of the power coming from a high voltage supply in a charging resistor, and additional energy is wasted in a double spark gap. Furthermore, the capacitive load of the coaxial corona reactor must be charged; this charging energy is typically much greater than the energy that is actually used in the corona reaction, and simply decays away into heat after each pulse without contributing to the pollutant destruction.
A similar approach, but with a different reactor geometry, is taken in Rosocha et al., "Treatment of Hazardous Organic Wastes Using Silent-Discharge Plasmas," Non-Thermal Plasma Techniques for Pollution Control, NATO ASI Series Vol. G34 Part B, Ed. by B. M. Penetrante and S. E. Schultheis, Springer-Verlag Berlin Heidelberg, 1993, pages 79-80, in which the corona discharge is established between parallel plates. This system also suffers from a poor specific energy due to inefficient pulse formation and non-recovery of reactor charging energy.
A pollutant destruction system using an inductor-capacitor (LC) resonant circuit for corona discharge generation is described in application Ser. No. 08/450,449, filed May 25, 1995, now U.S. Pat. No. 5,695,619, "Gaseous Pollutant Destruction Apparatus and Method Using Self-Resonant Corona Discharge," and assigned to Hughes Aircraft Company, the assignee of the present invention. The application discloses a single stage corona discharge reactor driven by an LC resonator circuit, which is efficient in converting high-voltage pulse energy to corona discharges.
High voltage pulses are very effective in destroying hydrocarbons (HC) and carbon monoxide (CO), but do not facilitate the reduction of nitrogen oxides (NO.sub.x) into diatomic nitrogen (N.sub.2) and oxygen (O.sub.2). Experiments have shown that using high voltages (up to 12 kV) may even produce some additional NO.sub.x. On the other hand, low voltage pulses are highly efficient in reducing NO.sub.x, but are very poor at oxidizing HC. Therefore, depending on the treatment desired, a wide range of voltages levels and frequencies may be required.
High voltage and high frequency electricity must be supplied to a corona discharge reactor to generate a corona discharge. The voltage required is usually in the range of about 5-20 kV, and the frequency required is usually in the range of about 5 to 15 MHz. A series resonant inverter with a feedback control loop for generating the required waveform is described in U.S. Pat. No. 4,757,432. Spark gap circuits have been used for generating high voltage pulses for corona discharge, and are described in A. Mizuno et al., "NO.sub.x Removal Process Using Pulsed Discharge Plasma," IEEE Transactions on Industry Applications, vol. 31, 1995, pages 957-962, and T. Fujii et al., "Pulse Corona Characteristics," IEEE Transactions on Industry Applications, vol. 29, 1993, pages 98-102.